水稻条斑病菌rsmA基因在致病性中的功能分析

Functional analysis of rsmA in pathogenesis of Xanthomonas oryzae pv. oryzicola

RsmA属于CrsA/RsmA蛋白家族成员,是一类RNA结合蛋白,作为一类全局性的转录后调控因子,调控碳代谢、生物膜形成、游动性以及致病性相关基因的表达等。同源性搜索结果显示,水稻条斑病菌(Xanthomonas oryzae pv.oryzicola,Xoc)RS105中存在rsmA基因,其与白叶枯病菌(Xanthomonas oryzae pv.oryzae,Xoo)PXO99A的rsmAXoo同源性为100%。但是,r smAXoc基因在致病性中的功能未知。本研究构建了Xoc的rsmA缺失突变体RΔrsmA。寄主水稻和非寄主烟草接种结果显示,RΔrsmA在水稻上仍具有致病性,在非寄主烟草上也能够激发HR反应,这些结果与已鉴定的Xoo rsmA突变体表型不一致。但是,与野生型菌株相比,RΔrsmA在感病水稻上的毒性显著降低;在丰富和贫乏的培养基中,RΔrsmA的生长能力也明显减弱。其他毒性相关表型的测定结果显示,与野生型菌株相比,RΔrsmA在半固体培养基上的游动能力减弱,生物膜形成能力增强,胞外多糖产量明显降低,胞外蛋白酶的活性增加。这些结果暗示在Xoc中rsmA为重要的毒性相关基因,在Xoo和Xoc中RsmA在致病性中的功能存在一定的差异,RsmA下游调控基因的鉴定可能为解析其在2个水稻致病变种中功能的差异提供线索。

RsmA belongs to a conserved family of RNA binding proteins （CrsA/RsmA family） functioning as a global post transcriptional regulator and has been demonstrated to regulate carbon metabolism, biofilm formation, motility, and the expression of pathogenicity genes. Homology search showed that there was a rsmA gene in Xoc strain RS105, which shares a 100% identity with rsmAXoc gene of a Xoo strain PXO99A. However, its function in Xoc remains unclear. In this work, we constructed a deletion mutant of rsmA gene in Xoc RS105 strain named as RΔrsmA. Inoculation assays showed that RΔrsmA can cause pathogenicity on host rice, and trigger a hypersensitive response on non host tobacco, which exhibits a different phenotype from that of the rsmAXoo mutant. However, compared with the wild type strain, significantly reduced virulence symptoms on susceptible rice were observed in the RΔrsmA mutant, and bacterial growths in nutrient rich and poor medium were also weakened. In addition, the RΔrsmA mutant showed weaker ability of swimming motility on semisolid medium, increased biofilm, decreased level of extracellular polysaccharide, but was enhanced in secreting extracellular proteases when compared with the wild type strain. These results indicate that rsmA is essential for the virulence of Xoc and there are some differences in functions of RsmA as virulence regulators in Xoo and Xoc. Therefore, identification of downstream genes regulated by rsmA may provide clues to elucidate the different roles in pathogenicity in the two pathovar strains of Xanthomonas oryzae.